Ultrasonic flow meters are often used to measure the flow rate of a fluid in a conduit. Different flow meters are based on several different physics principles. The most commonly used techniques are transit time, Doppler and cross correlation tag. All three techniques measure collective effects caused by the total fluid flow inside the conduit which are then analytically or empirically corrected by a factor to derive the mean flow rate of the flow inside the conduit. Typically, the correction factor is flow rate dependent and invalid if the flow profile is not symmetrical. However, Doppler techniques can be designed such way that signal propagation from wall to wall can be selectively analyzed or scanned. See U.S. Pat. Nos. 3,564,912; 6,732,595; 6,931,945 and Published U.S. Patent Application Nos. US2005/0245827; US2005/0241411; and US2005/0011279 all incorporated herein by this reference.
In all ultrasonic flow meters, transducers transmit signals to and receive signals from the fluid inside of a conduit. The transducers can be wetted or clamp-on. The wetted method involves a direct contact of the front face of the transducer with the fluid generally though a pipe nozzle. The clamp-on technique involves clamping the transducers to the outside of the conduit and beneficially does not change the integrity of the conduit surface. In the most common application, where the conduit is circular in cross section, high accuracy flow measurement by the transit time technique often involves the use of a multi-path technique with one pair of transducers per path is applied. It is preferred that a minimum of one path be located in the chordal location to measure undeveloped flow. Undeveloped flow generally does not have high symmetry in its profile and therefore cannot be well characterized by existing theories. However, the nozzles for the chordal path are generally more difficult to fabricate than for a diameter path. Misalignment and mislocations can contribute to inaccurate flow measurements. Furthermore, in the case of clamp-on style transducers, it is difficult to obtain a useful chordal path without changing the integrity of the existing pipe surface.
Thus, transducer pairs located on chordal paths through the flow cannot generally be of the clamp-on type and/or involve difficulties in mechanical design and construction. Some flow meter systems are difficult to set up for use on existing conduits. Misalignment and other factors can contribute to inaccurate flow measurements. Importantly also, the accuracy of a given flow meter often depends on whether or not the flow is symmetric or asymmetric. Additionally, for asymmetric flow, using range gated Doppler techniques to measure flow rate and obtaining a profile correction factor by scanning wall to wall of the conduit requires at least two paths and time-consuming scanning, which can compromise measurement accuracy.